Adequate uterine blood is required for normal growth and development of the fetus. Blood flowing to the uterus provides the blastocyst, embryo and fetus with all the necessary nutrients and oxygen which are required for growth and development. Since arterial blood pressure remains relatively constant during pregnancy, changes in uteroplacental blood flow are directly related to decreases in uterine vascular resistance. A significant portion of this change is due to the development of a new parallel circuit, the placenta. However, it is also thought that locally produced vasodilator agents are responsible for a significant portion of the increase in blood flow especially in late gestation. Recently, many cardiovascular scientists have turned their interest to the role of the endothelial cells in regulating blood flow in organs throughout the body. Endothelial cells produce several very potent vasodilators including endothelial derived relaxing factor (EDRF) and PGI(2). EDRF is thought to be the vasodilator nitric oxide, released during the conversion of L-arginine to L-citrulline, a reaction catalyzed by the calcium calmodulin dependent nitric oxide synthetase. This enzyme can be blocked by analogues of L-arginine (i.e., L-nitro arginine, L-mono-methyl arginine, etc.) an effect which is reversible by large doses of L-arginine but not D-arginine. The present application is based on exciting preliminary data (page 40) which demonstrates that estrogen induced increases in uterine blood flow are mediated by EDRF (NO) and that blockade of EDRF (NO) synthesis results in a 30% reduction in uteroplacental blood flow in late term sheep. Studies are outlined which will evaluate the role of EDRF in regulating systemic arterial blood pressure as well as uterine vascular resistance in normal pregnant and nonpregnant sheep and umbilical vascular resistance in the fetus. We will determine if nitric oxide synthetase (NOS) activity is increased in systemic and uterine vascular smooth muscle during pregnancy and determine if estrogen can induce NOS activity in the nonpregnant systemic and uterine vasculature. Studies are also planned which will determine if the nitric oxide breakdown product, nitrate, is elevated in the uterine and umbilical circulation in pregnancy and increases during gestation. We will also determine if estrogen administration increases systemic and uterine venous levels of plasma nitrate and cGMP. Finally, the ability of EDRF to modulate organ blood flow distribution as well as systemic and uterine vascular responses to endogenously occurring vasoconstrictors (norepinephrine, angiotensin II, serotonin and endothelin-1) in pregnant and nonpregnant sheep will be determined. These experiments should further clarify the role of endothelial derived relaxing factors in the regulation of systemic and uterine blood flow throughout pregnancy. Investigating their interactions with endogenous vasoconstrictors will increase our understanding of the physiological basis of vascular regulation and its pathologic deviation.